I wanted to create a simple Battery DC PSU. There I want to have +3,+6,+7.5 and +12.

In this PSU should be
-2x(2xAA battery holder)
-1x(Single AA battery)
-1x(9V battery)

Then I just started inserting everything in LTSpice http://img6.imageshack.us/img6/1084/ltspicemoregrounds.png
There it is. The only problem is that I need the 12V ground and the 3-7.5V ground. But LTSpice wont let me use both. It doesn´t make a difference if I connect for ex. a LED to the 3-7.5V ground but it seems like it is grounded to 12V or in the opposite way :/

Now I want to ask you, is this even possible in LTSPice? Or should I remain on one voltage supply for each schematic, but how should I use 2 or more different projects together if there can only be one ground?

Here's one way to approach something like that; use net labels instead, and use the common net label for a 2nd ground:

You can't label a single node more than one name.
If nodes are separated by a component, then they are separate nodes, and can be labeled differently.

One trick is to use a very low value resistor on a node that you want to have two names; there will be a voltage drop across the resistor, but it will usually be negligible.

Note that once you declare a net name, you don't have to run wires all over the schematic to get a signal somewhere; just paste another copy of the net name where you need it. That will help to make your schematics much easier to understand. Having power and ground lines running everywhere just makes it harder to understand.

Use the "COM" symbol, available under the Net Name dialog. Then, connect a 99Meg resistor between COM and GND, as no current flows between each side it makes them isolated but they assume the same potential.

In most (as far as I know, in all) versions of Spice, a ground has to be defined in order for the simulation to work.

Other common lines distinct from the actual Earth have to be called something else, but they do have to have some DC relationship to it, possibly defined by a very big resistance if they would be effectively floating in the real world.

Some versions of Spice have special symbols for "grounds" other than the master ground, otherwise connectors labelled something appropriate like "chassis" or "Vee" could be used.

Finally, however much difficulty you have with English, you are not doing too badly.
One thing you are not doing is writing "bad language": this is an English idiom meaning swearing, profanity.
Somebody who makes mistakes with English grammar, syntax or word usage would be said to use bad English.

Thank you very much SgtWookie, tom and Adjuster , also the two lines are to show how my power supply will be. I want to have 2 different cables, So I have a better view. I know it would work if I simply stick it in to the 3V and then the 9VGND